CODEWITHSUNDEEP
cfilememorystoragestructure
valon
valon

Reputation: 467

How do I store structures that are read from a file in C?

I came up with a project to work on learning C and I have kind of hit a wall. The project is simply a card game where the player has two sets of cards, one set is a deck of a set size and the other is their collection of cards that can be as big as it needs to be. The card structure is as follows:

struct card {   
    char name[256];
    char special[256];
    char type[100];
    char rarity[100];
    int points;
};

I then have a file of the collection called coll.txt

first card goes here 50
second card goes here 70
...

I then have a (sloppy) function that reads from the file and stores it into a temporary card:

void read_into_collection(FILE *f) {
    char *file_text;
    char *token;
    int i;
    struct card temp;

    file_text = (char *) malloc(sizeof(struct card));

    while(fgets(file_text, sizeof(struct card), f)) {
        for(i = 1, token = strtok(file_text, " "); token; i++, token = strtok(NULL, " ")) {
            switch (i) {
            case 1:
                strcpy(temp.name, token);
                break;
            case 2:
                strcpy(temp.special, token);
                break;
            case 3:
                strcpy(temp.type, token);
                break;
            case 4:
                strcpy(temp.rarity, token);
                break;
            case 5:
                temp.points = atoi(token);
                break;
            default:

                i = 0;
                break;
            }
        }
    }


    free(file_text);
}

so by the time i = 6 I am ready to move the temporary card to the collection and read the next card into the temp variable and so on. But how do I do this? I am having trouble figuring out what collection should actually be. At first I thought:

struct card *collection = (struct card *) malloc(number_of_cards * sizeof(struct card));

But, if I am correct, malloc() returns a pointer to a chunk of memory and the memory is not sequential like an array, so I cannot increment the pointer to store cards.

I also tried counting the number of lines in the file (each line is a card) and then making an array of that size but I get an error that the value is not constant.

What is the best way to go about storing these cards as a collection? I would just make the collection a really large array but I feel like this situation comes up often in projects and would rather learn how to handle it rather than take the easy way out.

Upvotes: 2

Views: 227

Answers (2)

djechlin
djechlin

Reputation: 60848

But, if I am correct, malloc() returns a pointer to a chunk of memory and the memory is not sequential like an array, so I cannot increment the pointer to store cards.

False. It is sequential. You can use malloc() to create an array of anything:

mystruct* ptr = (mystruct*) malloc(sizeof(mystruct) * numberOfStructs)

for(int i = 0; i < numberOfStructs, i++) {
    ptr[i].setSomeInfo(x);
}

This is the standard way to do it in C.

Upvotes: 3

JoeH-Java
JoeH-Java

Reputation: 126

I used to program in C a lot more, one of my favorites. Here is my answer.

The way you've used malloc did allocate an array

struct card *collection = (struct card *) malloc(number_of_cards * sizeof(struct card));

Each call to malloc will return a pointer to a different area in memory. But one call to malloc always returns a contiguous block.

If number of cards is known, you could use that to allocate an array and then access it like so

//added some code to prevent overflow
collection[i].name[255] = 0;
strncpy(collection[i] .name, token, 255);

What if number of cards is not known. Then do a linked list. That is primary use of a linked list, storing the contents of a collection whose size is unknown.

Upvotes: 1

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